CDMA communication system and method

ABSTRACT

In CDMA communications, a base station detects a difference between a reference phase of a spreading code and a phase of a received signal of each terminal station, and feeds a phase jump signal PJ-i representative of the phase difference back to each terminal station. After timing acquisition, the reception operation is performed using the reference phase, and a shift amount between the phase of a received signal and the reference phase is fed back to each terminal station as phase synchronization control information PC-i. Each terminal station coarsely adjusts the phase of the spreading code in accordance with the phase jump signal PJ-i, and thereafter finely adjusts the phase of its transmitting signal in accordance with the phase synchronization control information PC-i. It is therefore possible to synchronize the phases of the signals transmitted from terminal stations and received at the base station. Accordingly, an orthogonal code is used for spreading on the reverse links.

This is a continuation application of U.S. Ser. No. 08/888,603, filedJul. 7, 1997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a communication system using CDMA (CodeDivision Multiple Access) and a control method of tracking the phases ofspreading codes.

2. Description of the Related Art

CDMA is used for multiplexing a plurality of communication channels onthe same frequency band by using a plurality of different spectrumspreading codes.

A base station spreads a transmitting signal to each terminal station bymultiplying the transmitting signal by a spreading code specific to eachcommunication channel, and transmits a multiplexed signal in the form ofa plurality of mixed spread signals of communication channels by using asingle carrier frequency. Each terminal station de-spreads a receivedsignal by multiplying the reception signal by the code (de-spreadingcode) specific to its communication channel which is the same as thespreading code used by the base station, to thereby derive only acorrelated signal of its channel. In this case, other channel signalsare not correlated and become noise components because differentspreading codes and de-spreading codes are used. The level of noisecomponents can be lowered to a degree not to interfere withcommunications.

Attention has been paid to CDMA as a method of considerably improvingthe use efficiency of communication frequencies. CDMA has been adoptedin the U.S.A. as a standard (IS-95) of digital cellular mobilecommunication systems and is now in practical use in this nation. TheIS-95 standard uses orthogonal codes as spreading codes on a forwardlink for signal transmission from the base station to each terminalstation.

Examples of orthogonal codes are shown in FIG. 11 being indicated at W0,W1, W2 and W3. An orthogonal code has the nature that a sum of theproducts of two arbitrary codes in a code group within an orthogonalunit becomes zero.

FIG. 10 shows a radio communication system including of a plurality ofbase stations 401 (401-1 to 401-j) interconnected by a wired network anda plurality of terminal stations 402 (402-1 to 402-n) distributed withina communication area of each base station. According to the IS-95standard, the base station assigns specific orthogonal codes Wi (i=1 ton) to a plurality of terminal stations (or communication channels)within the communication area of the base station. The base stationspreads a signal or data destined for a terminal station i with anorthogonal code Wi specific to the terminal station and transmits it.The terminal station de-spreads the received signal with the orthogonalcode Wi specific to it so that signal components of all other channelsare removed during the de-spreading process and they do not becomeinterference signals.

Such communication systems, which use spectrum spreading with orthogonalcodes for communications from a base station to terminal stations overforward links, are disclosed, for example, in U.S. Pat. No. 5,103,459.

In order to utilize the advantages of orthogonal codes, it is necessaryto ensure perfect synchronization of orthogonal code timings between asubject reception channel and other reception channels during thede-spreading process. If there is any shift of timings of orthogonalcodes on a plurality of multiplexed channels, orthogonality is lost sothat received signal components of other reception channels becomeinterference signals relative to the signal of the subject receptionchannel and its signal-to-noise (S/N) ratio is degraded.

Even if each terminal station spreads transmitting data by usingorthogonal codes, the transmitting signal of each channel is receivedasynchronously at the base station because each terminal stationtransmits it independently over the reverse link. Therefore, thetransmitting signals of respective channels become interference signalsand a reception S/N ratio is degraded. Orthogonal codes withoutsynchronization used as spreading codes as in the above case limit thenumber of connection channels. For example, in order to realize areception S/N ratio of 10 dB, the number of connection channels viareverse links becomes about one tenth of the number of forward links.

For the above reason, a conventional CDMA communication system usesorthogonal codes only for one-to-N transmission links in order to easilyto synchronize timings of orthogonal codes between channels, i.e., onlyfor forward links from a base station to terminal stations. For N-to-onetransmission reverse links over which a plurality of terminal stationstransmit signals independently, non-orthogonal codes such aspseudo-noise (PN) codes have been used for spreading transmissionsignals.

JP-A-7-254867 discloses a CDMA communication system in which a basestation feeds a delay information on the channel back to a mobileterminal station, but does not refer to use of an orthogonal codetherein.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a CDMA communicationsystem and method capable of receiving good quality signals at both baseand terminal stations.

Another object of the present invention is to provide a CDMAcommunication system and a control method of tracking the phases ofspreading codes, capable of increasing the number of terminal stationsaccessible to the base station at the same time.

The present invention is applied to a CDMA communication systemcomprising a base station (main station) and a plurality of terminalstations (subsidiary stations), the base station providing a pluralityof channels to forward and reverse links through CDMA (code divisionmultiple access) and each terminal station being provided incorrespondence with each channel constituted of a pair of forward andreverse links.

In order to achieve the above objects, in one aspect of the presentinvention, the base station feeds information, which indicates a phasedifference between a phase of a received signal detected at each channelon a reverse link and a reference phase of a de-spreading code at saidbase station, back to the corresponding terminal station via thecorresponding forward link of the channel. Each terminal stationsynchronizes a phase of a spreading code of a transmitting signal on thereverse link with the reference phase at the base station, in accordancewith the phase difference information received at the correspondingforward link of the channel. Therefore, an orthogonal code can be usedas the spreading code of a transmission signal for both the reverse andforward links.

More specifically, according to the aspect of the invention, a receptioncircuit of each terminal station is provided with an orthogonalgenerator. The phase of an orthogonal code generated by the orthogonalgenerator is controllable in accordance with phase synchronizationcontrol information received from the base station on the forward link.Each channel signal on the reverse link can therefore be received at thebase station in an orthogonal state.

In order to synchronize the phases of orthogonal codes of respectiveterminal stations, the base station measures, for example, a phasedifference between the reception reference phase and the phase of areception signal of each terminal station, at each channel on thereverse link. A phase synchronization control signal generated inaccordance with the measured phase difference is fed back to eachterminal station. Each terminal station derives the phasesynchronization control signal of its own from the signal sent from thebase station, and in accordance with this derived control signal, thephase of the orthogonal code for the transmitting signal is controlled.

According to another aspect of the invention, when a terminal station isadditionally connected, the measurement results of the reception phaseat the base station are supplied to the terminal station. In accordancewith the measurement results, the terminal station sets the phase of thespreading code for the transmission signal to a predetermined phase.

The foregoing and other objects, advantages, manner of operation andnovel feature of the present invention will be understood from thefollowing detailed description when read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a first embodiment of a base stationconstituting a communication system of this invention.

FIG. 2 is a block diagram showing the details of a modem 106-i (i=1 ton) shown in FIG. 1.

FIG. 3 is a block diagram showing a first embodiment of a terminalstation of a communication system of this invention.

FIG. 4 is a block diagram showing a second embodiment of a base stationconstituting a communication system of this invention.

FIG. 5 is a block diagram showing the details of a modem 116-i shown inFIG. 4.

FIG. 6 is a block diagram showing the details of a modem 116-i (i=2 ton) shown in FIG. 4.

FIG. 7 is a block diagram showing a third embodiment of a base stationconstituting a communication system of this invention.

FIG. 8 is a block diagram showing the details of a modem 126-i (i=1 ton) shown in FIG. 7.

FIG. 9 is a block diagram showing a third embodiment of a terminalstation constituting a communication system of this invention.

FIG. 10 is a diagram showing a communication system to which theinvention is applied.

FIG. 11 is a diagram showing examples of orthogonal codes used forspectrum spreading.

FIG. 12 is a diagram showing an example of another communication systemto which the invention is applied.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the invention will be described, by taking a radiocommunication system constituted of base stations 401 and terminalstations 402 as shown in FIG. 10 as an example.

FIG. 1 shows the first embodiment of the structure of the base station401. In the first embodiment, at the base station, a desired receptionphase is set as a reference phase, and a transmission phase at eachterminal station is adjusted to be equal to the reference phase.

A signal received at an antenna 111 is input via a circulator 110 to aradio frequency circuit 109 where the received signal is converted intoa spectrum spread signal Rx on a baseband. The spread signal Rx is inputto a plurality of modems 106-i (i=1 to n) provided in correspondencewith terminal stations under communications.

At each modem 106-i, the input signal Rx is demodulated throughde-spreading and decoded to derive a transmitted signal (received data)of each terminal station. As will be later detailed with reference toFIG. 2, each modem 106-i performs two steps of de-spreading by usingpseudo noises PNr specific to each reverse link generated from areception pseudo noise (PN) generator 211 and by using an orthogonalcode Wi assigned to each terminal station and generated from anorthogonal code generator 212. Each modem 106-i includes an acquisitioncircuit 214 which operates for timing acquisition of a spreading code(hereinafter called "an acquisition searching mode") and a receivedphase state decision circuit 213 which operates for phase trackingcontrol after the timing acquisition (hereinafter called "a phasetracking control mode").

After the completion of the timing acquisition of a spreading code inthe acquisition searching mode, the acquisition circuit 214 outputsacquired phase information SP-i (i=1 to n). In the phase trackingcontrol mode, the decision circuit 213 compares the acquired phase whilea de-spreading process for the received signal Rx is performed inparallel, and outputs phase difference information PD-i (i=1 to n).

The acquired phase information SP-i (i=1 to n) and phase differenceinformation PD-i (i=1 to n) output from each modem 106-i (i=1 to n) areinput to a reverse link phase synchronization controller 103. Inaccordance with the contents of the acquired phase information SP-i (i=1to n), the reverse link phase synchronization controller 103 generatesphase jump information PJ-i (i=1 to n) of each terminal station, and inaccordance with the contents of the phase difference information PD-i(i=1 to n) it generates a phase synchronization control instruction PC-i(i=1 to n) of each terminal station.

Transmitting data 101 to each terminal station is mixed by acorresponding modem 106-i (i=1 to n) with the phase jump informationPJ-i or phase synchronization control instruction PC-i of the terminalstation selectively supplied from the reverse link phase synchronizationcontroller 103, and subjected to an encoding processing and a modulatingprocessing through spectrum spreading. This spectrum spreading isperformed in two steps by using pseudo noises PNf specific to eachforward link generated by a pseudo noise (Tx-PN) generator 104 and byusing an orthogonal code Wi specific to each terminal station generatedby an orthogonal code generator 105. The signal modulated throughspectrum spreading is output as a transmitting signal Tx-i (i=1 to n).

Transmitting signals Tx-i (i=1 to n) of respective terminal stations aresequentially added by cascaded adders 107, and thereafter converted by aradio frequency circuit 108 into a signal on the transmission frequencyband, and transmitted from the antenna 111 via the circulator 110.

FIG. 2 shows an example of the structure of the modem 106-i (i=1 to n)shown in FIG. 1.

The transmitting data 101 and the phase jump information PJ-i (i=1 to n)or phase synchronization control instruction PC-i (i=1 t on) are mixedby a frame composition block 201, and thereafter encoded (includingerror correction and the like) by an encoder 202. The encoded signal ismultiplied (first spectrum spreading processing) by the orthogonal codeWi assigned to the destination terminal station by a first multiplier203, and then multiplied (second spectrum spreading processing) by theforward link pseudo noises PNf by a second multiplier 204. This spectrumspread signal is output as a transmitting signal Tx-i.

The received signal Rx is input to a first multiplier 206 to bemultiplied (first spectrum de-spreading processing) by the orthogonalcode Wi generated by the orthogonal code generator 212. The orthogonalcode Wi is the same as the orthogonal code Wi used for spectrumspreading of the received signal Rx by the terminal station. An outputof the multiplier 206 is input to a second multiplier 207 to bemultiplied (second spectrum de-spreading processing) by the forward linkpseudo noises PNf generated by the pseudo noise (PN) generator 211.

An output of the multiplier 207 is input to an accumulator 208 whichaccumulates outputs of the multiplier 207 during a predetermined periodand outputs the accumulation results to a decoder 209 and theacquisition circuit 214. The signal accumulation period of theaccumulator 208 is changed with the type of operation mode. For example,the accumulation period is an eight-symbol period for the acquisitionsearching mode, and a one-symbol period for the phase tracking controlmode. On the assumption that data of consecutive (all) 1's istransmitted from a terminal station during the acquisition searchingmode and the de-spreading results of the received signals areaccumulated during the eight-symbol period at the base station, a gainof a correlation value can be increased by 9 dB so that the timingacquisition probability can be improved.

In an asynchronous state between the phases of the spreading code of theterminal station and that of the base station on the reverse link, i.e.,in the acquisition searching mode, the acquisition circuit 214 decidessynchronization based upon the results of the de-spreading outputs fromthe accumulator 208. During this period, a selection switch SW is turnedto the acquisition circuit side so that a control signal output from theacquisition circuit 214 is input to the PN generator 211 and orthogonalcode generator 212 of the received phase state decision circuit 213.

When it is determined from an output of the accumulator 208 that thephases of the spreading codes supplied to the multipliers 206 and 207are asynchronous with the phase of the spreading code of the receivedsignal Rx, the acquisition circuit 214 outputs a phase updateinstruction to the PN generator 211 and orthogonal code generator 212 inorder to shift the phases thereof by a predetermined amount. If it isdetermined that the phases become synchronous, then the acquisitioncircuit 214 calculates a difference between the phases of the PNgenerator 211 and orthogonal code generator 212 at that time for thereference phase, and outputs the difference as acquired phaseinformation which is notified to the terminal station. Data 102 outputfrom the decoder 209 during the acquisition searching mode (during anasynchronous period) is processed as invalid data.

When it is determined that the spreading codes become synchronous, theacquisition circuit 214 makes the selection switch SW to be turned to areference value generator side (in FIG. 2, this value is indicated as"0"). In this state, the phases of the PN generator 211 and orthogonalcode generator 212 are set to the reference phase, and the circuitoperation transits to the phase tracking control mode. During the phasetracking control mode, the de-spread received signal output from theaccumulator 208 is subjected to a decoding processing such as an errorcorrection processing by the decoder 209, and thereafter it is picked upas valid received data 102.

The received phase state decision circuit 213 surrounded by a brokenline in FIG. 2 is used for determining the phase of the received signalRx. The received signal Rx is subjected to a de-spreading processing byusing spreading codes having a half-chip forwarded phase and a half-chipdelayed phase relative to the phases of the spreading code (PN andorthogonal code) used by the multipliers 206 and 207 for de-spreadingthe received signal Rx. A difference between sums of respectivede-spread results (correlation values) is output as the phase differenceinformation PD-i.

In the decision circuit 213 shown in FIG. 2, outputs from the PNgenerator 211 and orthogonal code generator 212 are passed throughrespective cascaded half-chip delay circuits 210 of two stages, andspreading codes are generated having +/- half-pitch phases relative tothe phase of the spreading code, intermediate between the two delaycircuits. The spreading code having a forwarded phase is supplied tomultipliers 206' and 207' and the spreading code having a delayed phaseis supplied to multipliers 206" and 207", thereby to be multiplied bythe received signal Rx. The de-spread results of these two series ofcircuits are accumulated during a predetermined period by accumulators208' and 208", similar to the accumulator 208 for the received data.

FIG. 3 shows an example of the structure of the terminal station 402.

A signal received at an antenna 301 is input via a circulator 302 to aradio frequency circuit 303 where it is converted into a spectrumspreading signal on the baseband. The spectrum spreading signal on thebaseband is supplied to a first multiplier 304 of a reception circuit(demodulating circuit) to be multiplied (first de-spreading processing)by an orthogonal code Wi assigned to the terminal station and generatedby an orthogonal code generator 313. An output of the first multiplier304 is supplied to a second multiplier 305 to be multiplied (secondde-spreading processing) by forward link pseudo noises PNf generated bya pseudo-noise (PN) generator 312. The PN generator 312 is set withnoise patterns that are the same as the pseudo noises PNf specific tothe forward link generated at the base station.

An output of the second multiplier 305 is input to an accumulator 306which accumulates signals from the second multiplier 305 during apredetermined period. The accumulated signal is decoded (including errorcorrection and the like) by a decoder 307 and supplied to a framede-composition block 308.

The frame de-composition block 308 separates an output of the decoder308 into received data 309, and phase jump signal PJ-i or phasesynchronization control instruction PC-i. The phase jump signal PJ-i isinput to an orthogonal code generator 318 and a pseudo noise generator321 respectively of a transmission circuit (modulating circuit). Inaccordance with the contents of the phase jump signal PJ-i, the phasesof the orthogonal code Wi and pseudo noises PNr used for spreadingtransmitting data, are coarsely adjusted. The phase synchronizationcontrol instruction PC-i is input to a transmission phase controller315. In accordance with the contents of the phase synchronizationcontrol instruction PC-i, the transmission phase controller 315 outputsa control signal PS-i which is used for fine adjustment of the phases ofthe orthogonal code Wi and pseudo noises PNr.

Synchronization acquisition and tracking operations for PN andorthogonal code used by the de-spreading processing at the receptioncircuit are performed respectively by an acquisition circuit 314 and aDLL (Delay Lock Loop) circuit 310 surrounded by a broken line in FIG. 3.Similar to the received phase state decision circuit 213 of the basestation, the DLL circuit 310 supplies the outputs of the PN generator312 and orthogonal code generator 313 to half-chip delay circuits 311 oftwo stages to obtain spreading codes having half-chip forwarded anddelayed phases relative to the spreading codes (PN and orthogonal code)used for the de-spreading of received data.

In an asynchronous state (in a timing acquisition state) of the phasesbetween the received signal and de-spreading codes, the phases of the PNgenerator 312 and orthogonal code generator 313 are controlled by aphase updating instruction supplied from the acquisition circuit 314 viaa switch SW. When the phase synchronization is acquired, the PNgenerator 312 and orthogonal code generator 313 are connected to a loopfilter 325. In this state, the reception signal is de-spread bymultipliers 304' and 305' with the spreading code having a half-chipforwarded phase and by multipliers 304" and 305" with the spreading codehaving a half-chip delayed phase. The phases of PN and orthogonal codeare controlled so that the de-spread results of two forwarded anddelayed phase series to be obtained by accumulators 306' and 306",become equal.

In the transmitting circuit, transmission data 316 is input to anencoder 317 to be encoded (including error correction and the like), andthereafter spectrum-spread by two multipliers 320 and 322. The firstmultiplier 320 multiplies the encoded transmitting data by theorthogonal code Wi assigned to the terminal station to perform a firstspectrum-spread modulation. The next second multiplier 322 multiplies anoutput of the first multiplier 320 by reverse link pseudo noises PNr toperform a second spectrum-spread modulation. In this embodiment, theorthogonal code Wi generated by an orthogonal code generator 318 andpseudo noises PNr generated by a pseudo noise generator 321 are suppliedvia delay circuits 319 and 319' to the multipliers 320 322. The signaldelay amounts at these delay circuits are controlled by the controlsignal PS-i output from the transmission phase controller 315 to therebyfinely adjust the phases. An output signal of the second multiplier 322is converted into a signal on a transmission frequency band by a radiofrequency circuit 323 and transmitted from the antenna 301 via thecirculator 302.

In the above embodiment, it is possible to synchronize at the basestation the phase of a received signal from any terminal stations witheach other, by the phase jump function to be performed when theacquisition searching mode transits to the phase tracking control modeat the base station and by the transmission phase control to be executedat each terminal station during the phase tracking control mode.Accordingly, the advantageous performance of orthogonal codes can bepositively utilized and transmitting signals from terminal stations canbe prevented from being interfered with each other.

As above, in the first embodiment, a desired reception phase is set as areference phase by the base station, and the transmission phase of eachterminal station is synchronized with this reference phase. After thesynchronization acquisition, the phases of a PN generated by thereception PN generator 211 and an orthogonal code generated by theorthogonal generator 212, respectively shown in FIG. 2, are fixed to thereference phase set at the base station, and at this reference phase,de-spreading of the received signal is performed. For example, as thereference phase, the phases of the orthogonal code and PN at the basestation for the forward link may be used.

Next, the second embodiment of the invention will be described. FIG. 4shows the structure of the base station 401. Constituent componentscorresponding to those shown in FIG. 1 are represented by identicalreference numerals.

The structure and operation of the base station of the second embodimentare similar to those of the first embodiment. The point of difference isthat the phase of a transmitting signal from a terminal station receivedat one of the modems (in this example, a first modem 116-1) is used asthe reference phase to control the phases of transmitting signals fromother terminal stations.

Referring to FIG. 4, the modem 116-1 has a DLL circuit and de-spreads areceived signal while controlling the phases of the PN and orthogonalcode so as to synchronize them with a received signal from the terminalstation. In accordance with the phase of the PN and orthogonal codegenerated when the synchronization is acquired, the modem 116-1generates spreading code synchronization timing information CT. Inaccordance with this information CT, the other modems 116-i (i=2 to n)set the reference phase of their reception spreading codes (PN andorthogonal code) to perform the de-spreading processing and receivedphase state decision during the phase tracking control mode.

FIG. 5 shows an example of the first modem 116-1 shown in FIG. 4.

The points of difference from the modem shown in FIG. 2 are that thephase control of the PN generator 211 and orthogonal code generator 212to be executed by the reception phase state decision circuit 223 isperformed by using the control signal from the acquisition circuit 214similar to the DLL circuit 310 shown in FIG. 3 during the acquisitionsearching mode, and performed by a feedback control of an output of theloop filter 215 during the phase tracking control mode, and that in thetransmission circuit, feedback of the phase synchronization controlinformation to the terminal station is not performed but the values "0"are set to the phase jump signal PJ-1 and the phase control informationPC-1 supplied to the terminal station.

The phase of a received signal when the acquisition circuit acquires asynchronization is set to the loop filter 215. By using this phase as areference, the phases of the PN generator 211 and orthogonal codegenerator 212 are controlled through feedback. The phases of thesegenerators are sent to the other modems 116-i (i=2 to n) as thespreading code synchronization timing information CT.

FIG. 6 shows an example of the structure of the modem 116-i (i=2 to n)other than the first modem 116-1 shown in FIG. 4.

When the acquisition circuit 214 acquires synchronization during theacquisition searching mode, a difference between the current phases ofthe PN generator 211 and orthogonal code generator 212 and the referencephase indicated by the spreading code synchronization timing signal CTis used as the acquired phase information SP-i. After thesynchronization acquisition, the PN generator 212 and orthogonal codegenerator 212 of the phase state decision circuit 224 execute thefeedback control by using as the reference phase the phase representedby the spreading code synchronization timing signal CT. Different fromthe first modem 116-1, these other modems feed the acquired phaseinformation SP-i and phase difference information PD-i back to theterminal stations as the phase jump information PJ-i and phasesynchronization control information PC-i, respectively.

By using the above-described structure and control operation of the basestation and by using as the reference phase the phase of a receivedsignal from the terminal station corresponding to the first modem 116-1,synchronization control of received signals of the other terminalstations is executed. Therefore, received signals of all terminalstations reach the base station in an orthogonal state with each other.

Next, the third embodiment of the invention will be described. In thisembodiment, during the acquisition searching mode, the forward linkphase of a spreading code on the terminal station side is changed toperform the timing acquisition of the spreading code. In the followingdescription, it is assumed that the reference phase on a reverse link atthe base station is equal to the forward link phase.

FIG. 7 shows the structure of the base station 401 according to thethird embodiment of the invention. Constituent components correspondingto those of the first embodiment shown in FIG. 1 are represented byidentical reference numerals.

The operation of the base station 401 during the phase tracking controlmode is the same as the first embodiment, and the operation during theacquisition searching mode is different. In this embodiment, informationsupplied from each modem 116-i (i=1 to n) to the reverse link phasesynchronization controller 103 is only the phase difference informationPD-i (i=1 to n) generated during the acquisition searching mode. Theacquisition searching control information SC-i acquired during theacquisition searching mode is supplied, as shown in FIG. 8, directly tothe frame composition block 201 in each modem 126-i.

FIG. 8 shows an example of the structure of the modem 126-i. As apparentfrom the comparison to FIG. 2, a received phase state decision circuit233 of the modem 126-i of this embodiment performs a de-spreadingprocess of a received signal Rx by using always-fixed spreading codes(PN and pseudo noises) output from the PN generator 211 and orthogonalcode generator 212, without receiving an external control signal. Thisis because each terminal station transmits a signal while changing thephase of the spreading code during the acquisition searching mode. Thebase station has no function of actively acquiring synchronization whilechanging the phase of the spreading code. The phase of a PN code PNrgenerated by the PN generator and the phase of an orthogonal codegenerated by the orthogonal code generator are both fixed to apredetermined reference phase. When a synchronization of the phase ofthe received signal Rx with the reference phase is acquired, thereceived data 102 is made valid.

Similar to the first embodiment, the accumulation period of anaccumulator 238 inputted with de-spreading results of the receivedsignals Rx is changed with the operation mode. For example, this periodmay be an eight-symbol period for the acquisition searching mode and aone-symbol period for the phase tracking control mode. In thisembodiment, during the acquisition searching mode, the accumulator 238discards the de-spreading results during a short period, e.g.,one-symbol period, necessary for phase updating at the terminal station,and actually accumulates during the period of seven symbols. On theassumption that during the acquisition searching mode, data ofconsecutive (all) 1's is transmitted from a terminal station and thede-spreading results during the period in which seven symbols areaccumulated without using the transmitted bits during the phase updatingperiod, a gain of a correlation value can be increased by 8.5 dB so thata timing acquisition probability can be improved. The accumulationtiming of the de-spreading results by the accumulator 238 is determinedfor each modem by taking into consideration a delay time for a signaltransmitted from a terminal station to reach the base station.

During the acquisition searching mode, the de-spreading results of thereceived signal Rx are input to the acquisition circuit 234 to determinefrom the correlation value of the de-spreading results whethersynchronization is acquired or not. The determination results are outputas acquisition searching control information SC-i (i=1 to n) to theframe composition block 201. The acquisition searching controlinformation SC-i is transmitted to the terminal station, as anacquisition searching mode termination notice signal whensynchronization is acquired or as an acquisition searching modecontinuation notice signal when synchronization is not acquired.

FIG. 9 shows an example of the structure of the terminal station 402 ofthe third embodiment. Constituent components corresponding to those ofthe first and second embodiments shown in FIG. 3 are represented byusing identical reference numerals.

In this embodiment, the frame de-composition block 308 derives theacquisition searching control information SC-i (i=1 to n) mixed with thereception data transmitted from the base station, and inputs it to theorthogonal code generator 318 and PN generator 321 of the transmissioncircuit. In accordance with the contents of the acquisition searchingcontrol information SC-i, the orthogonal code generator 318 and PNgenerator 321 control the phases of the orthogonal code Wi and PN codePNr. For example, while the acquisition searching control informationSC-i indicates a continuation of the acquisition searching mode, thephases of the orthogonal code Wi and PN code PNr are sequentially slid,for example, by a half-chip synchronously with the operation timing ofthe accumulator 306 of the reception circuit. Upon reception of theacquisition searching mode termination signal, the phases of theorthogonal code Wi and PN code PNr are slid back by an integer multipleof a half-chip at the next phase shift timing to thereby synchronize thephases with the phase of the inverse spreading code at the base station.This phase slide-back takes into consideration a phase slide of thespreading code at the base station during the delay time required forthe acquisition searching mode termination signal to reach the terminalstation after the synchronization decision at the base station.Immediately after the completion of the acquisition searching mode, theoperation transfers to the phase tracking control mode to perform thephase tracking similar to the first and second embodiments.

According to the first to third embodiments described above, a signal ofeach channel on the forward and reverse links can be made orthogonal sothat a signal of a subject channel with high quality can be receivedwithout being interfered with by other channels on both the terminal andbase station sides. Since an orthogonal code is used for both theforward and reverse links, the number of terminal stations capable ofcommunicating with the base station can be increased and terminalstations which are the same in number as the processing gain can beaccommodated at a maximum.

If the invention is applied to a cellular mobile communication system,the phase control is performed during a period shorter than a changeperiod of the communication conditions (such as fading or Dopplerfrequency shift) at the terminal station. In the above embodiments,although the same orthogonal code is assigned to each terminal stationfor both the forward and reverse links, different orthogonal codes maybe assigned to the forward and reverse links.

In the above embodiments, the structures of the base and terminalstations have been described assuming that the invention is applied tothe radio communication system shown in FIG. 10. The invention may beapplied to other communication systems. For example, as shown in FIG.12, the invention is applicable to a CDMA two-way communication system(CDMA/C system) in which a base switching center 403 and a plurality ofcell stations 404 of a personal handyphone system (PHS) areinterconnected by a CATV network. In this system shown in FIG. 12, thebase switching center 403 corresponds to the base station 401, each cellstation 404 corresponds to the terminal station 402, and the antenna 111of the base station and the antenna 301 of the terminal station arereplaced by a cable.

The invention may also be applied to other radio communication systems,for example, wireless local loop (WLL) systems in which base andterminal stations are installed at fixed locations.

It is to be understood that the above-described embodiments are merelyillustrative of the principles of the invention and that many variationsmay be devised by those skilled in the art without departing from thespirit and scope of the invention. It is therefore intended that suchvariations be included within the scope of the claims.

What is claimed is:
 1. A communication method in a spread spectrumcommunication system for performing communication between a base stationand terminal stations, comprising the steps of:transmitting, in eachterminal station, a first signal spread with a spreading code generatedby a spreading code generator; performing, in the base station, a timingacquisition of the first signal from each terminal station andtransmitting a difference information between the first signal and areference on the timing acquisition; setting, in each terminal station,the phase of the spreading code generated by the spreading codegenerator in accordance with the first difference information andtransmitting a second signal spread with the spreading code having theset phase; performing, in the base station, a tracking of the secondsignal from each terminal station and transmitting second differenceinformation between the second signal and the reference; setting, ineach terminal station, a delay amount of a delay circuit in accordancewith the second difference information, the delay circuit adjusting thephase of the spreading code of a signal transmitted to the base station.2. A communication method according to claim 1, wherein the spreadingcode is selected from among a plurality of orthogonal codes which areorthogonal to each other.
 3. A communication method according to claim1, wherein the base station calculates a first de-spread result of thesecond signal with the spreading code whose phase is forwarded byhalf-pitch and a second de-spread result of the second signal with thespreading code whose phase is delayed by half-pitch; andthe base stationoutputs the second difference information as the difference between thefirst and second de-spread result.
 4. A terminal station for performingcommunication with a base station, comprising:a spreading code generatorwhich generates a spreading code assigned to the terminal station andhaving a phase set in accordance with a first difference informationtransmitted from the base station, wherein the first differenceinformation indicates a difference on the timing acquisition between areference and the first spectrum spreading signal which is transmittedfrom each terminal station and received by the terminal station; amodulator which spreads a transmitting data with the generated spreadingcode; and a delay circuit which adjusts a phase of the spreading code ofa spectrum spreading signal spread by the modulator in accordance with asecond difference information transmitted from the base station, whereinthe second difference information indicates a difference during trackingbetween the reference phase and a second spectrum spreading signal whichis transmitted from each terminal station and received by the basestation.
 5. A terminal station according to claim 4, wherein thespreading code is selected from among a plurality of orthogonal codes.6. A terminal station for performing communication with a base stationcomprising:a spreading code generator which generates a spreading codeassigned to the terminal station, the spreading code generator slides aphase of the spreading code by a predetermined amount until receiving afirst difference information transmitted from the base station, whereinthe first difference information indicates a completion of timingacquisition of the transmitting signal from the terminal station; amodulator which spreads a transmitting data with the generated spreadingcode; and a delay circuit which adjusts a phase of the spreading code ofthe transmitting signal spread by the modulator in accordance with asecond difference information transmitted from the base station, whereinthe second difference information indicates a difference during trackingbetween a reference and the transmitting signal which is transmittedfrom each terminal station and received by the base station.
 7. Aterminal station according to claim 6, further comprising a delaycircuit which delay the generated spreading code and input the delayedspreading code to the modulator,wherein a delay amount of said delaycircuit is set in accordance with the second different information.
 8. Aterminal station according to claim 6, wherein the transmitting data arefixed as consecutive symbols before the completion of timingacquisition.
 9. A base station for performing communication withterminal stations via a plurality of forward channels and reversechannels comprising:a demodulator which de-spreads a received signal oneach reverse channel; an acquisition circuit which performs a timingacquisition of the received signal and detects a first differencebetween a spreading code corresponding to a reverse channel for thereceived signal and a reference; a phase state decision circuit whichperforms a phase tracking of the received signal and detects a secondphase difference between the spreading code corresponding to a reversechannel for the received signal and the reference; a phasesynchronization controller which generates a first and a seconddifference information respectively indicating the first and the seconddifference; a modulator which spreads the first or the second differenceinformation with a spreading code corresponding to a forward channel fortransmitting the first or the second difference information.
 10. A basestation according to claim 9, wherein said spreading code is selectedfrom among a plurality of orthogonal codes.
 11. A communication methodin a spread spectrum communication system for performing communicationbetween a base station and terminal stations, comprising the stepsof:transmitting, in each terminal station, a first spectrum spreadingsignal; performing, in the base station, a timing acquisition of thefirst spectrum spreading signal from each terminal station andtransmitting a first difference information which indicates a differencebetween a spreading code for the first spectrum spreading signal and areference phase on the timing acquisition; setting, in each terminalstation, a phase of a spreading code generated by a spreading codegenerator in accordance with the first difference information andtransmitting a second spectrum spreading signal spread with thespreading code generated by the spreading code generator; performing, inthe base station, a phase tracking of the second spectrum spreadingsignal from each terminal station and transmitting a second differenceinformation which indicates a difference between the second spectrumspreading signal and the reference during the phase tracking; setting,in each terminal station, a delay amount of a delay circuit inaccordance with the second difference information, the delay circuitadjusts a phase of a spreading code of a spectrum spreading signaltransmitted to the base station.
 12. A communication method according toclaim 11, wherein the spreading code is selected from among a pluralityof orthogonal codes which are orthogonal to each other.
 13. Acommunication method according to claim 11, wherein the base stationcalculates a first de-spread result of the second spectrum spreadingsignal with the spreading code for the second spectrum spreading signalwhose phase is forwarded by half-pitch and a second de-spread result ofthe second spectrum spreading signal with the spreading code for thesecond spectrum spreading signal whose phase is delayed by half-pitch;andoutputs the second difference information as the difference betweenthe first and second de-spread result.
 14. A communication method in aspread spectrum communication system for performing communicationbetween a base station and terminal stations, comprising the steps of:inan asynchronous state between a first spectrum spreading signal and areference, wherein the first spectrum spreading signal is transmittedfrom each terminal station and received by the base station, performing,in the base station, a timing acquisition of the first spectrumspreading signal and transmitting a first difference information whichindicates a difference between the first spectrum spreading signal andthe reference on the timing acquisition; and setting, in each terminalstation, a phase of a spreading code generated by a spreading codegenerator in accordance with the first difference information; in asynchronous state between a second spectrum spreading signal and thereference, wherein the second spectrum spreading signal is transmittedfrom each terminal station and received by the base station, performing,in the base station, a tracking of the second spectrum spreading signaland transmitting second difference information which indicates adifference between the second spectrum spreading signal and thereference during the tracking; and setting, in each terminal station, adelay amount of a delay circuit in accordance with the second differenceinformation, the delay circuit adjusts a phase of a spreading code of aspectrum spreading signal transmitted to the base station.
 15. Acommunication method according to claim 14, wherein the spreading codeis selected from among a plurality of orthogonal codes which areorthogonal to each other.
 16. A communication method according to claim14, wherein the base station calculates a first de-spread result of thefirst spectrum spreading signal with the spreading code for the firstspectrum spreading signal which phase is forwarded by half-pitch and asecond de-spread result of the first spectrum spreading signal with thespreading code for the first spectrum spreading signal which phase isdelayed by half-pitch; andoutputs the second difference information asthe difference between the first and second de-spread result.